Yeah, mIRC shouldn't do this by default. Imagine connecting to a server and joining 10 channels each having a full banlist. If your server supports 30 bans, that's 300 ban lines you have to receive before you can start anything else, and getting the nick list of all those channels is bad enough. If you need to know then send out the the /mode +b and use the raw remote to trigger your script instead of on join.

Cheers,
GregMo